Abstract – the NMR application to the analysis of solids requires the high speed spinning of the sample under test, up to speeds in tens of thousands of rpm. These speeds are achieved with microturbines. The speed and pressure measurement and control developed for a commercial application are described in this work.

Keywords – IEEE Keywords

I. INTRODUCTION

Nuclear Magnetic Resonance (NMR) is a powerful technique for the determination of molecular structure of substances through the analysis of the spectrum of resonant frequencies (or precession) of the alignment of atoms in the direction of a large external magnetic field. The resonant frequency is influenced by the local field on which the neighboring atoms have an impact. The analysis of these spectra, that are composite graphs of the resonances of all the types of atoms in the substance under test, allows for the determination of the structure of molecules (i.e. distances and angles between atoms). For example, soluble substances result in molecules randomly tumblig whose corresponding atoms are resonating at the same frequency. This behavior of soluble substances results in averaging of any local field irregularity thus leading to pretty sharp spectral lines. Spinning of the sample pushes this characteristic even further. Furthermore, a technical problem in NMR probes lies in creating an external magnetic field uniform enough to allow the measurement of the resonant frequencies with the required accuracy. However, a slight disuniformity of the magnetic field in the sample tube (i.e. 20 p.p.m.) can be compensated by spinning the sample in order to average the differences. Finally a very significant improvement in the signal to noise ratio of the receiver coil is achieved through cryogenically cooling the receiver coil itself and

part of the related electronics.

Notice that the less uniform the field, the greater the

spinner speed should be. At present, NMR has been more successful with liquids (approximately 80% of NMR tests) or materials dissolved in solvents than with solids, while with solids or mixed samples a lack of resolution persists. For liquid samples and high quality magnets, it is generally sufficient to spin the sample at few revolutions per second (r.p.s.). For solid samples, instead, the main testing difficulty lies in the lack of rapid molecular tumbling and diffusion to average out chemical shift anisotropy and dipolar couplings. Hence the spectral lines are normally broad and unresolved (often hundreds of ppm in width). Notice that human tissue samples for diagnostic analysis fall in this category.

Many techniques have been developed to improve the resolution in NMR of solids. Most recent approaches include extremely rapid spinning of the sample at the “Magic Angle” (MAS), the zero of the second Legendre polynomial, 54.7 ̊, with respect to the external field. In fact, if the rotational rate is fast compared to chemical shift anisotropies and dipolar couplings (in units of Hz), the resolution is dramatically improved – often by two or three orders of magnitude. In practice, in order to obtain satisfactory results, the sample has to spin fast enough that the nuclei cannot relax too much during a single revolution. Even when the spinning is not fast enough to satisfy the above conditions, substantial improvements in resolution are generally obtained from the combination of MAS and multiple pulse methods. Similar resolution problems are encountered in liquids of inhomogeneous samples, such as tissues and mixtures of liquids and solids, because the susceptibility variations throughout the material. Here, relatively slow MAS is often effective in improving the spectral resolution of the liquid species by several orders of magnitude. Spinning the sample at a constant rate in the range tens of thousands r.p.s., as opposed to 20 r.p.s. of liquids, achieves the averaging effect. This spinning rate often must be maintained for long time intervals, in the order of hours. In addition, speed variations sometimes have to be limited within two